1. Field of the Invention
This invention relates to an exposing apparatus for transferring a pattern formed, for example, on a reticle (mask) onto a wafer, and particularly to an exposing apparatus provided with an alignment apparatus for detecting the relative position of the reticle (mask) and the wafer.
2. Related Background Art
When a semiconductive element, a liquid crystal display element or the like is to be manufactured by the lithography process, use is made of a projection exposing apparatus in which a pattern image on a photomask or a reticle (hereinafter generically referred to as the "reticle") is projected onto a wafer as a photosensitive substrate through a projection optical system. Generally, when a semiconductive element or the like is to be manufactured, a plurality of layers of circuit patterns are formed in superposed relationship with each other on a wafer. In this case, it is necessary to highly accurately effect the alignment of a pattern formed in the preceding step and a pattern image to be transferred this time, i.e., the relative alignment of the wafer and the reticle and therefore, an alignment apparatus is provided in the projection exposing apparatus.
As a conventional alignment apparatus, there is known an alignment apparatus for setting an alignment mark (wafer marks) formed near each shot area on a wafer to a predetermined positional relation to thereby effect the alignment of the wafer and a reticle. In this case, there are wafer mark detecting methods of the TTL (through-the-lens) type and the TTR (through-the-reticle) type, and in the method of the TTL type, the alignment mark on the wafer is detected by alignment light incident from between the reticle and the projection optical system through the projection optical system.
On the other hand, in the method of the TTR type, alignment light for detecting the wafer mark is applied from above the reticle and the alignment light transmitted through the reticle is applied to the wafer mark on the wafer through the projection optical system. The alignment light reflected by this wafer mark is received by a light receiving element in the alignment apparatus via the projection optical system and the reticle.
In any of these methods of the TTL type and the TTR type, light having a wavelength differing from that of exposure light and weak in sensitizing property to a photosensitive material (such as photoresist) has been used as the alignment light so that the photosensitive material applied onto the wafer may not be sensitized. Ultraviolet light of e.g. wavelength 248 nm has been used as the exposure light, and He-Ne laser light of wavelength 633 nm has been used as the alignment light. In this case, as antireflection coating formed on each lens element of the projection optical system, use has been made of multilayer coating designed to have an antireflection characteristic to both of the exposure light and the alignment light.
In order that as in the prior-art exposing apparatus, the antireflection coating provided on the projection optical system may have the antireflection characteristic to not only the exposure light but also the alignment light (wavelength 633 nm), it is necessary to provide complicated multilayer coating, and this leads to the inconvenience that the design and manufacuture of the antireflection coating are difficult.
Particularly, in a projection optical system for excimer laser using, for example, excimer laser light of wavelength 248 nm as exposure light, of the coating material is very much limited. If under such limitation, an attempt is made to provide antireflection coating for excimer laser light of wavelength 248 nm and He-Ne laser light of wavelength 633 nm, the antireflection characteristic will generally be deteriorated as compared with antireflection coating of respective single color. Further, the antireflection coating in that case is complicated multilayer film and therefore, the thickness of the film becomes great and the irregularity of the film thickness becomes liable to occur, and the irregularity of transmittance becomes liable to occur.